A conventional technique will be described by taking, for example, a detector for PET (Positron Emission Tomography) which is a radiation detector. As a detector for PET, a radiation detector has been proposed, which has detector cells of small area arranged at pitches of about 1 mm for the purpose of improvement in spatial resolution. For securing an effective field of view size by using such detector cells arranged at small pitches, it is necessary to arrange the detector cells two-dimensionally, which requires a huge number of channels.
Generally, there are two types of methods for identifying which detector cells of the numerous detector cells have received radiation. The first is a method which reads in parallel all signals that can be acquired from the channels (see Patent Document 1, for example). The second is a method called Anger method which is realized through centroid calculations by analog circuitry (see Patent Document 2, for example). Conventionally, since it is necessary to lower the cost of the circuit portion in a commercial machine, Anger method is used which uses a PMT (photomultiplier tube).    [Patent Document 1]    Unexamined Patent Publication H8-68863 (pages 2-3, FIG. 1)    [Patent Document 2]    Unexamined Patent Publication H7-311270 (pages 2-3, FIGS. 2 and 4)